Transport: Maritime engineers are already embracing electric propulsion for ships—and electric planes could be next

ACROSS the road from a golf course and next to a verdant, cow-filled field in Whetstone, a village about as far from the sea as it is possible to get in England, there is a ship's engine-room in a barn. The area is dripping with history—Frank Whittle, one of the inventors of the jet engine, used a neighbouring shed for his project—but this is not some clanking historical curiosity, such as a steam engine rebuilt by an amateur enthusiast. The whirring gas turbine and whining motor being put through their paces in bucolic Leicestershire are at the cutting edge of maritime engineering. The electric drive being tested there could represent the next leap forward in ship design, as significant a technological shift as the one from sail to steam power in the 19th century.

Even since the first intrepid boatman thought to pick up a piece of wood to paddle against the current, boat propulsion has been a terribly mechanical affair. Galley slaves pulled on oars; river-boat steam engines turned paddles; and nuclear reactors boiled water to drive turbines connected to propellers on aircraft carriers and submarines. What makes the experimental engine room in Leicestershire so special is that it leaves out the bit that usually links the engine and propeller. Instead of a propulsion shaft connecting the two, the all-electric drive being tested uses the ship's engines (turbine or diesel) to burn fossil fuels to generate electricity, which is then routed down thick cables to an electric motor that drives its propellers.

The idea of using electricity to drive ships is not new. Almost a century ago, around the time of the emergence of modern ship propulsion, electric drives were seen as viable contenders to compete with the then-rising mechanical drives. In 1912 America's navy built an electrically powered collier, followed a few years later by a string of battleships which proved capable workhorses during the second world war. Some electric-drive warships used 20% less fuel than conventional geared-turbine vessels. But these early examples were large and unwieldy, and the idea was abandoned.

Its recent rebirth has been almost as rapid as its fall, helped by two related developments: power electronics capable of handling huge flows of current, and smaller, more powerful electric motors. These advances have allowed shipbuilders to reduce the size and weight penalties associated with electric drives. They have also made possible the development of totally integrated power systems, which make energy fungible: instead of having one engine dedicated to driving the ship and another one devoted to generating shipboard power, electricity from multiple sources can simply be routed to wherever it is needed at the time.

And that is why advanced navies such as Britain's and America's are now among the most enthusiastic and earliest adopters of electric-drive ships. As warfare has become more digital, the demand for electricity on board warships has increased. Radar, computers and combat systems now account for as much as 30% of the fuel burned on modern warships. And the demand for power could be about to jump dramatically. Some navies are already testing rail guns, which use huge amounts of electricity to produce a magnetic field which then accelerates projectiles to many times the speed of sound.

Even more futuristic and power-hungry applications are within sight, such as “direct energy” weapons that zap enemy ships and “electric armour” that vaporises incoming missiles. With such demands for power, some of it only for a fraction of a second, warship designers are keen to have a single system doing all manner of things. Think of the Enterprise in “Star Trek”, where power is diverted to the shields, weapons or warp drive as needed. “We're going with electric drive because of warfighting need,” says Rear-Admiral Kevin McCoy of the American navy. “We are almost at the limits of technology and affordability in making improvements in mechanical drives.”

Electric drives offer other advantages, too. By distributing engines and generators around a ship, designers can make it more resistant to damage. Maintenance is easier if some of the engines can be stopped without halting the ship. And electric drives can help to cut costs. Although they are more expensive, bigger, heavier and in theory less efficient than mechanical drives, they use much less fuel. This is because the diesel engines and gas turbines commonly used to power ships are most efficient when buzzing away constantly at close to their maximum output. Throttle them back even a little, and the amount of energy obtained for each barrel of fuel burned falls sharply. That does not matter for ships that potter along all day and night at the same speed from one port to another; their engines can be run at their most efficient setting all the time. But in the navy, few ships have that luxury.

By some estimates, American navy ships spend 80% of their time travelling at half speed, which requires barely one-eighth of the power needed to propel a ship at top speed. But this requires them to burn almost as much fuel as they would when going much faster. By using an electric drive of the sort that is purring away in Leicestershire, ships can fire up their engines one at a time and run each at its most economical throttle setting.

Take Britain's newest warships, the Type 45 destroyers, which will be powered by the system now being tested in Whetstone. Each one will have four engines, half as many as the ships they are replacing, and usually only one will be running, says Paul Norton of the Ministry of Defence. The savings can be huge. America's Congressional Research Service reckons that installing electric drives on naval ships can cut fuel use by 10-25%. The American navy, which already has a handful of electric-drive support ships, expects savings of close to 20% for future warships using the technology.

Divert all power to the disco

Perhaps surprisingly, many of these advantages also apply to cruise liners, which present designers with many of the same problems as warships. Cruise ships need huge amounts of power. Stephen Payne, the chief designer of the Queen Mary 2, reckons the ship—the world's biggest passenger liner when it was launched in 2003, and fitted with an electric-drive system—could supply enough electricity for a town of 700,000 people.

Much of this power is used to keep passengers happy, running air conditioners during hot Caribbean days, for instance, and powering discos and cinemas in the evenings. Then, once all the passengers have gone to bed, the power can be routed down to the propellers for a high-speed dash to the next port. The technology is also being applied in cargo shipping. Makhlouf Benatmane of Converteam, one of the leading designers of electric drives for ships, reckons that almost two-thirds of liquefied natural-gas carriers now being built have electric drives. These cost about 4% more than the steam turbines that are traditionally used, but offer fuel savings of 10% or more, he says.

Now that electrical propulsion is being taken seriously in ships, not to mention trains and cars, can electric drives defy gravity and remake air travel? In some ways they are already doing so. Manufacturers of civil and military jets are collaborating on projects to add more electrically powered devices to aircraft, such as landing gear and flaps. These are lighter and more reliable than the hydraulic and mechanical systems now in use and could draw power as needed from non-essential systems such as in-flight entertainment. Military jets, meanwhile, need ever more power for their radar systems or to jam and dazzle those of their enemies. And airborne lasers are on the way.

Electrical propulsion is much more difficult. But some small experimental aircraft are already flying with electric motors driving their propellers. They are generally powered by high-discharge lithium-polymer batteries, which are also being used in some electric cars. Fuel cells are another option. Boeing is testing an electrically powered light aircraft which uses both batteries and a fuel cell as power sources. Some gliders also have small electric motors as auxiliary propulsion systems. Just a small amount of electrical propulsion, carefully applied, can dramatically increase the amount of time a glider can stay in the air. And many unmanned aerial vehicles, not to mention remote-control aircraft flown by hobbyists, are electrically powered.

Yet all these electrical aircraft are small and have limited range. What of larger aircraft? Retrofitting a large airliner with electric motors instead of engines would not be feasible because the power-to-weight ratio of an electric motor cannot compete with that of a jet engine, and storing and generating the energy needed for a long-haul flight would not be possible given the shape and size constraints of existing aircraft. But a “blended wing”—an aircraft in which the fuselage is a flat, tail-less structure resembling a giant wing—could provide huge efficiency gains and may form the basis of future airliners. Coincidentally, this shape would also lend itself to centrally placed generators, which could be gas turbines or fuel cells, driving electric fans or pusher-propellers distributed along the back edge of the wing, or blended into the body.

These propellers could be driven by superconducting motors, which can generate three times the torque of a conventional motor of the same weight and power input, according to a paper published in August in the journal Superconductor Science and Technology by Philippe Masson and his colleagues at Florida State University. American researchers are working on superconducting technology for maritime propulsion, which would leapfrog the British electric-drive system. Just as the earliest aircraft owed much to boat-building techniques at the start of the 20th century, the same might be true of electric-drive aircraft in the 21st.